Polytypism in xonotlite Ca₆Si₆O₁₇(OH)₂

Occurrence, chemistry, crystal growth, technical applications, structure and polytypism of xonotlite Ca 6 Si 6 O 17 (OH) 2 are reviewed. Atomic coordinates of the three simplest ordered polytypes in modified Gard notation: Ma 2 bc (space group P 2/ a , a = 17.032, b = 7.363, c = 7.012 Å, β = 90.36°), Ma 2 b 2 c (space group A 2/ a , a = 17.032, b = 7.363, c = 14.023, β = 90.36°), and M 2 a 2 bc (space group P 1̅, a = 8.712, b = 7.363, c = 7.012 Å, α = 89.99° β = 90.36°, γ = 102.18°) were modeled from geometric principles based on the known structure of the M 2 a 2 b 2 c polytype (space group A1̅, a = 8.712, b = 7.363, c = 14.023, α = 89.99 β = 90.36, γ = 102.18°). Unique reflection arrangements in the reciprocal lattice, characteristic of each polytype, were defined as criteria to identify xonotlite polytypes on X-ray single-crystal photographs. Precession- and Weissenberg-photographs of a xonotlite from the Kalahari manganese field (Republic of South Africa) indicated the predominance of the (100) twinned M 2 a 2 b 2 c polytype, followed by the Ma 2 b 2 c polytype, and very low concentrations of the Ma 2 bc polytype. The M 2 a 2 bc polytype could not be identified which agrees with previous electron diffraction experiments on xonotlites from other localities. Diffuse streaks parallel to a * and less intensive ones parallel to c * on single-crystal photographs suggest the presence of additional disordered polytypes.